In order to meet the increasing needs for reducing the overall sizes, increasing the degree of precision and improving the reliability of apparatus for information-communication, audio-visual application, more and more number of brushless motors are being used in place of motors with brush. The motors with brush are also required to be compact and thin; accordingly, many of the conventional frame cases of a cylindrical shape are replaced by those of an oval shape among the core motors. Even among the coreless motors, frame cases of oval and square shapes have been developed, and motors having such frame cases are already in use in portable communication appratus. The battery-driven motors for use in portable apparatus have to meet stringent dimensional requirements in thickness direction; in addition, the power consumption must be low in view of battery life. It has become difficult to raise the efficiency of a motor further by using a high energy-density magnet among the motors having conventional oval or cylindrical structure.
Motors of the above category have a structure as shown in FIG. 58, FIG. 61 or FIG. 62. The structure is described in the following.
FIG. 58 is a cross sectional view showing a conventional inner-rotor type brushless motor.
As shown in FIG. 58, a cylindrical magnet 130 is fixed to a shaft 131 inserted through the central hole, one end of the shaft 131 is held by a bearing 133 provided on a frame case 132 while the other end is held by a bearing 134 provided on a bracket 136; thus an inner rotor is formed which is supported at both ends. The magnet 130 fixed to the shaft 131 which is rotatably supported by bearings 133, 134 is magnetized in the N and S poles. A cylindrical core 128 is provided with three salient poles, which are wound around with a coil 129. The magnet 130 is rotated by magnetic flux generated by electricity supplied to the coil 129.
A circuit board 135 is mounted with electronic components. Each of the salient poles of the core 128 is wound around with coil to form a three-phase coil of phase U, phase V and phase W, respectively. The above electronic circuit controls so as the phase of induced voltage generated at each of the three phases deviates relative to one another by 120. Thus, it is driven as a three-phase brushless motor.
FIG. 61(a) is a cross sectional view showing a conventional core motor having an oval cross sectional shape for use in a portable pager, FIG. 61(b) shows the motor sectioned by a plane perpendicular to the shaft.
As shown in FIG. 61(a) and FIG. 61(b), a core 142 made of stacked silicon steel sheets is fixed around a shaft 139, a resin insulator shaped to a same shape as the core is inserted in the core 142, and a rectifying terminal unit 144 is thrusted to the shaft 139. The core 142 is wound around with a coil 143, the electric conduction point of coil 143 is aligned to a specified position of the rectifying terminal unit 144 for soldering. An armature coil assembly of a motor is thus structured. The rectifying surface of rectifying unit 144 is lapped, and then the entire assembly of armature coil is washed. A sintered bearing 140 is fixed in the centre of frame case 137. In the frame case 137 which is shaped oval, an arc-shape magnet 138 is provided at the inside of each of the two arc-shape sides of the frame case 137, the inner side of the two magnets is magnetized so as to have opposit pole relative to each other. The shaft 139 of washed armature coil assembly is inserted to the sintered bearing 140, and a bracket 147 provided with a brush 146 and a sintered bearing 141 is affixed to the frame case 137 to complete a motor.
Magnetic flux comming out of the inner surface of one magnet 138 goes through core 142, enters into the inner surface of the other magnet 138, and returns to the initial magnet 138 via frame case 137. Namely, a magnetic circuit is formed on a plane perpendicular to the shaft 139. An imbalancing weight 148 is mounted fixed on the shaft 139.
As a result of rotation of the motor, the shaft 139 with the imbalancing weight 148 fixed thereon causes a vibration, which vibration is transmitted to the frame case 137 for the vibration of a portable pager.
FIG. 62(a) is a plane view of a coreless motor having a square cross section, for use in a portable communication apparatus. FIG. 62(b) is a cross sectional view of the coreless motor of FIG. 62(a).
A shaft 149 is fixed to a group of coreless coils 151 via a rectifier 150. A magnet 152 of empty-cylindrical shape is fixed to a housing 153 and is disposed in a space within the group of coils 151 with a certain clearance. Frame case 154 provided with a flat portion on the outer surface fixes the housing 153 carrying thereon the magnet 152 in a space inside the group of coils 151 securing a certain clearance, at the same time forms a magnetic circuit together with the magnet 152. Bearing 155 fixed in the housing 153 holds the shaft 149 rotatable. Brush 156 is electrically coupled with the group of coils 151 via rectifier 150. Imbalancing weight 157 is fixed onto the shaft 149.
The above described conventional structure may not fully meet the increasing needs for a compact and efficient motor that satisfies the prevailing desire in the industry for making an apparatus smaller. The problems are as follows.
With an inner-rotor type brushless motor having a cylindrical core 128, as shown in FIG. 58, if there is a dimensional restriction in a plane perpendicular to shaft 131 the winding work for coil 129 may face a difficulty unless a certain space is secured for the work, especially when the diameter of magnet 130 becomes small the winding work may face an extreme difficulty.
The space factor in the above described inner-rotor type brushless motor is not quite high, by the reason described below.
FIG. 59(a) is a cross sectional view of simplified magnetic circuit in an inner-rotor type brushless motor. FIG. 59 (b) shows the motor unrolled from inside of the core. Description is made in the following with reference to these drawings. Here, the leakage of magnetic flux is ignored to make the description simple.
An inverse number of the velocity shift rate .mu. is often used to represent the efficiency .eta. of a motor. A following Formula 1 is generally established with respect to the velocity shift rate .mu.: ##EQU1## where: .PHI. denotes effective magnetic flux of core, T is number of coil turns, R is coil resistance.
The effective magnetic flux .PHI. of core is expressed in the following Formula 2. EQU .PHI.=.pi.DLBg (2)
where: D denotes the outer diameter of rotor, L is length of rotor, Bg is the density of magnetic flux at gap.
The density of magnetic flux at gap Bg is expressed in the following Formula 3. ##EQU2## where: Br, .mu. r are called respectively residual flux density, recoil magnetic permeability. These are the constants specific to a magnetic material. Lm is thickness of magnet, Lg is air gap between magnet and core.
The relationship between the number of coil turns T and the coil resistance R is represented in Formula 4. ##EQU3## where; k denotes electric conductivity of coil, which is a constant determined proportionate to the space factor of coil. l represents average coil length per one turn, S is cross sectional area of coil. The l is represented by Formula 5, when the coil resistance is ignored. ##EQU4## where; Lc denotes height of coil, Dc is coil width.
By substituting the Formula 2, Formula 3, Formula 4 and Formula 5 for Formula 1, the following Formula 6 is obtained: ##EQU5##
There is no component representing the coil resistance R nor the coil turn counts T contained in the above Formula. It indicates that the efficiency does not change by a change in the specifications of coil winding.
In the above formula, when L, D are varied so as the volume of rotor(.pi.D.sup.2 L/4) remains constant, with other variables fixed, the relationship between the length/diameter of rotor(L/D) and the efficiency .eta. is as shown in FIG. 60. According to FIG. 60, the efficiency improves with a longer rotor, but the improvement curve saturates and converges towards a certain value. So, there is a limitation, with no further improvement any more.
With regard to an inner-rotor type brushless motor as shown in FIG. 58, the number of turns for a salient pole has to be increased to obtain a larger torque. This makes the salient pole larger, hence a larger outer diameter of a motor, rendering it difficult to make a moter smaller. Furthermore, a core which is comprised of salient poles wound around with coil disposed at a same interval has to be provided around the inner-rotor, which makes cross sectional shape of a motor round, or almost round. It is difficult to make a motor thin.
With regard to a core motor for portable pager as shown in FIG. 61, the reliability is inferior to a brushless motor because of the existence of a brush. If one tries to solve the the problems by making the motor shape oval, the coil winding faces a difficulty because of the small contour.
With regard to a coreless type brush motor for portable communication apparatus, as shown in FIG. 62, the reliability is inferior to a brushless motor, and an available output torque is smaller as compared with a core motor when the dimensions of a motor are reduced. Namely, when diameter of a motor is reduced smaller it becomes more difficult to realize an efficient and compact coreless motor, or such a coreless motor for generating vibrations. Furthermore, coils of such a motor are required to be wound with a thin wire, e.g. as thin as 0.01-0.02 mm, which means a deteriorated production yield rate during handling of the coils. These have been some of the factors which hamper the supply of inexpensive motors.